Isn’t is simpler to pump the energy into a market pool connecting Morocco to the Iberian peninsula and connecting Britain to mainland Europe through the tunnel with France? Creating a cable to power a place thousands of km away seems rather inefficient compared to connecting the power stations to a grid of cables which already exists.
The Chunnel has nothing to do with power from France to Britain. Power cables are laid separately on the sea floor. They don't need, and have no place, in transport tunnel.
I also think it would better to use existing grids, but it may be easier to run direct cable than get all the pieces upgraded.